1. Field of the Invention
The present invention relates to a sintered bearing and a production method therefor suitable in various rotating portions or motors mounted in, for example, information devices, office equipment, electrical appliances, construction equipment, and the like.
2. Related Art
Sintered bearings produced according to a powdered metal technique have been widely used for the above applications since they are easy and inexpensive to mass-produce, and in particular, the size of the final product is uniform. Sintered bearings are roughly produced by compacting a metallic raw powder into a bearing green compact, sintering the green compact into a sintered compact, then subjecting the sintered compact to a sizing process to finish into required size and shape.
Sintered bearings are commonly used as oil retaining sintered bearings in which a lubricating oil is impregnated in pores therein for lubrication. As oil retaining sintered bearings for precise high speed rotation motors used in, for example, spindle motors for HDDs (hard disk drives), there have been provided bearings in which grooves for generating dynamic pressure are formed on an inner peripheral bearing surface. The dynamic pressure is generated by increase in the pressure in a oil film formed between a rotating shaft and a bearing, and this realizes support of the rotating shaft in a non-contact manner with a low torque and highly precise rotation.
The oil retaining sintered bearings sometimes exude the oil and contaminate the surroundings, and the bearings are not easy to use in portions required to usually be clean, such as hinges of covers in note-type personal computers and portable telephones.
When oil retaining sintered bearings are used for guide bushes in transportation structures in construction equipment such as power shovels, the bearings are exposed to sand and water introduced therein. Therefore, bearings are, for example, sealed in cases to form waterproof means so as to improve corrosion resistance and reliability. However, the waterproof means require complicated structures, and maintenance is not easy since the case must be opened when the oil must be resupplied. Moreover, sintered bearings are washed before impregnation of a lubricating oil. When the detergents used for the washing remain in pores of the sintered bearings, noxious gases may be given off due to increase of temperature in use.